1. Field of the Invention
The present invention relates to a method of recording information on and reproducing information from a magnetooptical recording medium.
2. Description of the Prior Art
In the magnetooptical information recording and reproducing process, a light beam such as a laser beam is applied to a magnetooptical recording medium to form information recording pits or bubble magnetic domains where information is recorded, and the recorded information is read by a magnetooptical interaction known as the Kerr effect or the Faraday effect. To increase recording density of the magnetooptical recording medium, the size of information recording pits may be reduced. The resolution with which the recorded information can be reproduced from the magnetooptical recording medium is governed by the wavelength .lambda. of a reading laser beam applied to the magnetooptical recording medium and the numerical aperture N.A. of the objective lens through which the reading laser beam is applied to the magnetooptical recording medium.
Efforts have been made to increase the resolution beyond the above limitation, i.e., to accomplish super-resolution. One such attempt is disclosed in Japanese laid-open patent publication No. 3-93058. The disclosed process employs a magnetooptical recording medium having a multilayer magnetic film structure including a reproducing layer and a recording layer. To read information recorded on the magnetooptical recording medium, a reading laser beam is applied to the magnetooptical recording medium, and, utilizing a temperature distribution of the magnetooptical recording medium within the spot of the applied laser beam due to the relative movement of the reading laser beam and the magnetooptical recording medium, information pits or magnetic domains are copied from the recording layer to the reproducing layer only in a local high-temperature region within the laser beam spot. This process is referred to as "RAD-MSR" process.
However, no special consideration is given to a recording process. According to the usual recording process, the information that has already been recorded on the magnetooptical recording medium is erased, and then new information is recorded on the magnetooptical recording medium. Therefore, the time required to record new information is relatively long, and the device used to record new information is relatively complex in structure.
More specifically, when information is to be recorded on a magnetooptical recording medium, i.e., information pits or magnetic domains are to be formed on the magnetooptical recording medium, any existing information is erased from the magnetooptical recording medium by magnetizing the magnetooptical recording medium in a uniform direction perpendicular to a magnetic film on the magnetooptical recording medium, and then a light beam such as a laser beam is applied to heat a local region of the magnetooptical recording medium for forming information pits therein that are perpendicularly magnetized in a direction opposite to the above uniform direction, thereby recording the information in binary form. This magnetooptical recording process, however, cannot record desired information in a short period of time as any previously recorded information must be erased before new information is recorded.
In view of the above problem, there have been proposed overwriting recording processes which are free of any independent erasing procedure.
These proposed recording processes include a magnetic-field-modulated recording process which modulates an external field to be applied to a magnetooptical recording medium depending on an information signal and a double-head recording process which uses a recording head and an erasing head.
One magnetic-field-modulated recording process is disclosed in Japanese laid-open patent publication No. 60-48806, for example. According to the disclosed process, to record information on a recording medium with an amorphous ferrimagnetic thin film having an easily magnetizable axis extending perpendicularly thereto, a laser beam is applied to an overwrite region on the recording medium to heat the same, and a magnetic field with its polarity corresponding to the status of an input digital signal is applied to the overwrite region for thereby recording the information. However, attempts to record desired information on the recording medium at high speed with a high information transfer rate require an electromagnet that can operate at high frequencies of several megahertz. Such an electromagnet cannot easily be fabricated, consumes a large amount of electric energy, and dissipates a large amount of heat.
The double-head recording process needs an extra head, with the two heads positioned in spaced-apart relationship. The magnetooptical drive systems based on the double-head recording process are put under large burden during operation, are not economic, and do not lend themselves to mass-production.
To solve the above problems, there has also been proposed another overwriting recording method which record new information over previously recorded information on a recording medium having recording and reproducing layers. Such an overwriting recording method is disclosed in Japanese laid-open patent publication No. 63-52354, for example.
According to this overwriting recording method, a light beam is intensity-modulated by information to be recorded and applied to heat a thermomagnetic (magnetooptical) recording medium to switch the recording medium, depending on the information to be recorded, between a first heated state in which the recording medium is heated to a first temperature T.sub.1 that is higher than the Curie temperature T.sub.C1 of the reproducing layer and at which the magnetic moment of the recording layer is not reversed, and a second heated state in which the recording medium is heated to a second temperature T.sub.2 that is higher than the Curie temperature T.sub.C1 and which is high enough to reverse the magnetic moment of the recording layer. When the recording medium is cooled from the above first and second heated states, two magnetized states are formed for thereby recording binary information on the recording medium.
The above recording method allows desired information to overwrite the recording medium with a light beam such as a laser beam that is intensity-modulated by such desired information.